Don't quote me on this in a court of law but I'm pretty sure all of my current main sources of income (other than consulting) started life as side projects, including a site I sold for 5 figures, a technology I sold for 6 figures, and most of my current sites which provide a day to day income now (the biggest of which was set up as a side project to try and help promote a book I was writing at the time). A personal blog I started in 2003 as a laugh made mid 4 figures every month in 2008 with Adsense (but almost nothing now, bah). I started an e-mail newsletter as a side project in August last year.. now up to 12k subscribers across 2 newsletters and making a little money some weeks.

Side projects rock! They sniff out your true interests and true motivations. But.. I doubt they're for everybody, naturally.